I think this is a useful feature but I don't think it is a substitute for moderation. Some people engage in personal attacks when confronted with others' opinions they don't agree with. I think that should be unacceptable. Everybody should be entitled to express themselves without fearing that some kind of cyber lynch mob is going to attack them if they say something unpopular. I see this feature being mostly used by those who would do the personal attacking, and it might reduce the signal-to-noise ratio online, but I still think it's important to impose some base levels of mutual respect for fellow users.